Well-known examples of an inorganic amorphous fiber material are: slags discharged from various glass furnaces and blast furnaces; basic rocks such as basalt; clay minerals such as Kaolin; and mixtures of Bayer process alumina with quartz sand or silica sand.
Such inorganic amorphous fiber materials are discharged continuously from a cupola or electric furnace through its tap hole in the form of a fine flow and then elongated by means of an outer force such as an air jet so as to make fibers. Three typical fiber-producing methods are:
(A) a drawing method by which continuous glass fibers are produced by drawing a fiber material continuously from a nozzle;
(B) a throwing method or a spinning method by which fibers are produced by centrifugal force of a spinner which is a rotation disc for making fibers; and
(C) a blowing method by which fibers are produced by a gas flow such as an air jet at a high speed.
According to a conventional blowing method, a pressurized gas such as air is jetted out through nozzle so as to obtain a flow speed near sonic velocity. A molten fiber material flow is guided into such a gas flow so as to produce fine fibers by means of the gas jet's impact on the molten fiber material flow.
The conventional blowing method has at least three defects.
A first defect is that a very large energy is required in order to compress air for jetting purposes. For example, in case Al.sub.2 O.sub.3 --SiO.sub.2 ceramic fibers are produced, a molten fiber material flow is discharged from a melting furnace at a flow rate of 400 kg/hr, while an air pressure of several kilograms per cm.sup.2 is applied. In order to produce such an air pressure, the air compressor must have about 300 horsepower (255 kw). Therefore, the energy necessary for making fibers is 255 kwh/400 kg=0.64 kwh/kg. In comparison with this energy, the energy necessary for fusing an Al.sub.2 O.sub.3 --SiO.sub.2 material is about 1.8 kwh/kg. Thus, the fiber-making energy is not small.
A second defect of the conventional blowing method is to need a means for collecting fibers. Jet air flowing at a high speed near sonic velocity induces some ambient air into the collecting chamber so that the volume of air to be discharged from the collecting chamber is several times as large as that of the jet air. If the discharging capacity is small, the pressure in the collecting chamber becomes larger than that of the atmosphere so that some fibers may be discharged out of the collecing chamber. In such a case, a large filter device must be provided so as to separate the fibers from the air. Also, a dust collector must be provided so as to keep the ambient atmosphere clean. Therefore, the cost of equipment and operation thereof is large. For example, the electric power required for operating the filter device and the dust collector is 0.5 kwh/kg in total.
A third defect of the conventional blowing method is that the rate of making fibers is low. Some shots is produced in fibers so as to adversely affect the otherwise low heat conductivity of the ceramic fibers functioning as a heat insulating material.